strings: type assert in the generic nextind, prevind methods

[nsajko]

The type assertions are valid according to the doc strings of these functions in the case of AbstractString.

Should prevent some invalidation on loading user code.

Fixes #57605

[nsajko]

Decreases the number of invalidations on loading the following code from 1996 to 1472:

struct I <: Integer end
Base.Int(::I) = 7

[Moelf]

But why are these assertions needed in the first place, the method signature already fixed these them to be Int, seems weird

[nsajko]

But why are these assertions needed in the first place

For good return type inference, we only needed the assertion on n, as pointed out by @jishnub on Slack, the assertion on i was not necessary.

The current PR removes even the type assertion on n in favor of asserting at an earlier point, on the isvalid call: the type of n isn't known if the return type of isvalid isn't known.